1. Field of the Invention
The present invention relates to a top nozzle having on-off hold-down springs for a nuclear fuel assembly used in a nuclear reactor, thereby preventing the uplifting of the nuclear fuel assembly, and more particularly, to a top nozzle having on-off hold-down springs for a nuclear fuel assembly that has a two-stage elastic section such that a pushing force against the axial movement of the nuclear fuel assembly under normal conditions is optimized and at the same time a suppressing force against a drastic uplifting force of the nuclear fuel assembly under transient conditions is strengthened.
2. Background of the Related Art
A nuclear reactor is a device that artificially controls the chain reaction of the nuclear fission of fissile materials, thereby achieving a variety of use purposes such as the generation of heat, the production of radioisotopes and plutonium, the formation of radiation fields, or the like.
Generally, enriched uranium that is obtained by raising a ratio of uranium-235 to a range between 2% and 5% is used in a light water nuclear reactor. The uranium is molded to a cylindrical pellet having a weight of 5 g and is processed to a nuclear fuel used in the nuclear reactor. Numerous pellets are piled up to form hundreds of pellet bundles and then put into a cladding tube made of Zircaloy being at a vacuum state. After that, a spring and a helium gas are put thereinto, and a top end closure stopper is welded thereon, thereby making a fuel rod. The fuel rod is finally surrounded by a nuclear fuel assembly and then burnt up within the nuclear reactor through nuclear reaction.
The nuclear fuel assembly and the parts therein are shown in FIG. 1. FIG. 1 is a schematic view showing a general nuclear fuel assembly.
Referring to FIG. 1, the nuclear fuel assembly includes a skeleton comprised of a top nozzle 4, a bottom nozzle 5, guide thimbles 3, and a plurality of spacer grids 2, and a plurality of fuel rods 1 inserted longitudinally into an organized array by the spacer grids 2 spaced along the length thereof in such a manner as to be supported by means of springs (which are not shown) and dimples (which are not shown) disposed within the spacer grids 2. So as to prevent the formation of the scratches on the fuel rods 1 and the generation of the damage on the springs within the spacer grids 2 upon assembling the nuclear fuel assembly, thereafter, the fuel rods 1 have a locker applied thereon and are then inserted longitudinally into the skeleton of the nuclear fuel assembly. Next, the top and bottom nozzles are secured to the opposite ends of the nuclear fuel assembly, thereby finishing the assembling procedure of the nuclear fuel assembly. Then, after the locker of the finished assembly is removed, the distances between the fuel rods 1, the distortion of the nuclear fuel assembly, the total length thereof, and the dimension thereof are checked out, thereby finishing the manufacturing procedure of the nuclear fuel assembly.
Next, an explanation on the structure of the top nozzle 4 will be given with reference to FIG. 2, wherein the top nozzle 4 has a hold-down plate 20, a plurality of outer hold-down springs 30, a plurality of outer guide-tubular sleeves 40, a flow plate 10, and a center guide-tubular sleeve 50. Each of the outer guide-tubular sleeves 40 of the top nozzle 4 is connected at the lower portion thereof to each guide thimble 3 (see FIG. 1) of the skeleton and connected at the upper portion thereof to each insertion tube 6 in the reactor, thereby firmly fixing the nuclear fuel assembly in the reactor and ensuring the structural stability during the burn-up of the nuclear fuel.
In more detail, the nuclear fuel assembly receives a hydraulic uplift force generated by the coolant flow during the reactor operation, such that it is floated or vibrated. Further, the thermal expansion due to the temperature rising, the irradiation growth of the nuclear fuel guide thimbles due to the neutron irradiation for a long period of time, and the variation of the axial direction length by creeps are generated in the nuclear fuel assembly. Therefore, the mechanical and structural stability of the nuclear fuel assembly against the axial direction movements and the length variations thereof should be ensured, which is achieved by the top nozzle 4, specifically the outer hold-down springs 30 of the top nozzle 4.
In accordance with the designed shapes of the nuclear fuel assembly, there are provided several kinds of hold-down springs. Such the hold-down coil springs as shown in FIG. 2 are adopted in standard nuclear fuel assemblies generally used in Korea. Since the hold-down coil springs have a feature of operating only in an elastic section thereof, they should be designed to satisfy the elastic limits thereof.
The hold-down coil springs in the nuclear reactor ensure their elastic section under generally expected operation conditions, that is, under normal conditions, and if the uplift force is generated within the elastic section, the hold-down coil springs have to have a minimum elastic coefficient capable of gently absorbing the generated uplift force, thereby preventing the fuel rods from being bent or distorted due to the deviation of the nuclear fuel assembly from its original position. On the other hand, under transient conditions, that is, if a drastic uplift force is generated, the hold-down coil springs should have a predetermined elastic coefficient such that they are not compressed below their close contact height (at which the springs are not pressed anymore since no space between the coils of the springs exists).
In the conventional top nozzle having the hold-down coil springs, if the elastic coefficients of the springs are much lowered, the fuel rods are not sufficiently protected due to the limitation to the close contact height under the transient conditions, and contrarily, if the elastic coefficients of the springs are much raised, the springs are not elastically moved relative to the uplift force of the nuclear fuel assembly, thereby causing the fuel rods to be bent or damaged. Therefore, it is difficult to provide the springs having the elastic coefficient satisfying that the above-mentioned conditions.
Therefore, there is a need for the development of the top nozzle having the springs providing a minimum hold-down force requested under normal operation conditions and at the same time easily satisfying the limitation to the close contact height and the allowable stress reference.